Showing papers on "Magnetite published in 2000"

Novel Photocatalyst: Titania-Coated Magnetite. Activity and Photodissolution

Abstract: Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nano-magnetite particles. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electron−hole recombination. Interactions between the iron oxide core and the titanium dioxide matrix upon heat treatment were also seen as a possible cause of the observed low activities of these samples. Other issues considered include the physical and chemical characteristics of the samples, such as surface area and the presence of surface hydroxyl groups. Depending on the calcination conditions, these photocatalysts were found to suffer from varying degrees of photodissolution. Photodissolution tests revealed the greater the extent of the heat treatment, the low...

Free-Standing Layer-by-Layer Assembled Films of Magnetite Nanoparticles

Abstract: A new technique for preparation of free-standing ultrathin films is presented. These films were made by layer-by-layer (LBL) deposition process, which is utilized for coatings on solid substrates and colloids. A film composed of alternating layers of magnetite nanoparticles and poly(diallyldimethylammonium bromide) was assembled on cellulose acetate, which was subsequently dissolved in acetone. From the suspended state, the LBL film can be transferred onto any solid or porous support. The strength of the film was observed to significantly increase when every other layer of magnetite was replaced with a layer of alumosilicate sheets serving as a molecular framework for the assembly.

Magnetite defines a vertebrate magnetoreceptor

Abstract: The key behavioural, physiological and anatomical components of a magnetite-based magnetic sense have been demonstrated in rainbow trout (Oncorhynchus mykiss) Candidate receptor cells located within a discrete sub-layer of the olfactory lamellae contained iron-rich crystals that were similar in size and shape to magnetite crystals extracted from salmon Here we show that these crystals, which mapped to individual receptors using confocal and atomic force microscopy, are magnetic, as they are uniquely associated with dipoles detected by magnetic force microscopy Analysis of their magnetic properties identifies the crystals as single-domain magnetite In addition, three-dimensional reconstruction of the candidate receptors using confocal and atomic force microscopy imaging confirm that several magnetic crystals are arranged in a chain of about 1 microm within the receptor, and that the receptor is a multi-lobed single cell These results are consistent with a magnetite-based detection mechanism, as 1-microm chains of single-domain magnetite crystals are highly suitable for the behavioural and physiological responses to magnetic intensity previously reported in the trout

Mössbauer characterization of iron oxides and (oxy)hydroxides: the present state of the art

Abstract: Mossbauer spectroscopy is a powerful direct technique for the identification and quantification of iron oxides and (oxy)hydroxides in soils and sediments. However, further characterization with respect to structural properties such as crystallinity, Al substitution, stoichiometry, water content, etc. is rather limited. With some examples of synthetic and natural goethite and hematite sample series it is illustrated that the hyperfine parameters depend on much more structural features than the Al content and crystallinity alone. Neither the Morin transition in hematite nor the Verwey transition in magnetite is directly applicable for analytical purposes in natural samples.

Preparation of amorphous magnetite nanoparticles embedded in polyvinyl alcohol using ultrasound radiation

Abstract: Uniformly dispersed amorphous nanoparticles of magnetite in polyvinyl alcohol have been obtained by ultrasound radiation. The properties of the as-prepared composite material were characterized by various analytical methods. We have found that the magnetite particles that were 12–20 nm in diameter were very well dispersed in the PVA. The magnetization measurements establish that the composite material is superparamagnetic in nature.

Magnetic properties of hydrothermally synthesized greigite (Fe3S4)- II. High- and low-temperature characteristics

Abstract: SUMMARY The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700°C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diVraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20‐30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100‐120 mT at 4 K, and the remanent coercive force increases from approximately 50‐80 mT at room temperature to approximately 110‐180 mT at 4 K.

Rock magnetic properties related to thermal treatment of siderite: Behavior and interpretation

Abstract: Detailed analyses of rock magnetic experiments were conducted on the oxidation products of high-purity natural crystalline siderite that were thermally treated in air atmosphere. Susceptibilities increase sharply between 400° and 530°C indicative of some new ferrimagnetic mineral phase generation. Both a drop (between 540° and 590°C) on the heating cycle and a dramatic increase (from 590°C to 520°C) on the cooling cycle occurred and are well consistent with the characteristic of magnetite. A distinct Hopkinson-type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700°C over and over. It has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite-bearing specimens is changed to a normal magnetic fabric during incremental heating over 410°–490°C. This is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals. A strong chemical-viscous remanent magnetization could be produced during siderite oxidation in an external field. Rock magnetic experimental results show that magnetite, maghemite, and hematite are the transformation products of high-temperature oxidation of siderite in air. Maghemite was not completely inverted to hematite even at temperature as high as 690°C during incremental thermal treatments. The mineral transformation processes were confirmed by conventional optical microscopic observation, X-ray diffractometry and Mossbauer spectroscopic analyses. These results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks. We conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments. Furthermore, it is clear that great care should be exercised in thermal demagnetization of siderite-bearing rocks in paleomagnetic, magnetic anisotropy, and rock magnetic studies.

Low-temperature magnetic properties of pelagic sediments (Ocean Drilling Program Site 805C): Tracers of maghemitization and magnetic mineral reduction

Abstract: Magnetic properties of pelagic sediments from the western equatorial Pacific (Ocean Drilling Program (ODP) Site 805C) have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth. A saturation isothermal remanent magnetization (Mrs) was given to the samples by applying a strong magnetic field (2.5 T) at 20 K after cooling both in the absence (zero field cooling, ZFC) and presence of a 2.5 T magnetic field (field cooling, FC). The thermal demagnetization of both MrsZFC and MrsFC was measured from 20 to 300 K. No pronounced Verwey transition was observed in the data from samples shallower than the iron redox boundary. Below the boundary the Verwey transition is visible for all samples studied. We interpret this behavior as reflecting low-temperature oxidation (maghemitization) of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary. The low-temperature oxidation may occur both at the water-sediment interface and deeper in the suboxic zone of sediments as a result of bacteria-mediated processes, including the reduction of nitrate compounds. MrsZFC and MrsFC thermal demagnetization curves of all samples diverge for the entire temperature range measured. The divergence may be caused by the low-temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system. Together, maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low-temperature properties of the sediment studied. As a result, low-temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments. The changes in effective oxidation state below the iron redox boundary detected by low-temperature methods may be useful in the study of some paleoenvironmental processes.

Influence of Fe3+/Fe2+ Ratio on the Crystallization of Iron‐Rich Glasses Made with Industrial Wastes

Abstract: The influence of the Fe3+/Fe2+ ratio on the crystallization of iron-rich glasses was investigated in this study. The glass batches were made from two hazardous industrial wastes: mud (goethite and jarosite) originating from the zinc hydrometallurgical process and electric arc furnace dust (EAFD). Glass compositions were prepared by adding different percentages of carbon powder. The crystallization process was investigated by a combined thermogravimetry/differential thermal analysis technique, in air or nitrogen atmospheres, using powder and bulk glass samples. The crystalline phases formed, i.e., pyroxene and spinels, and their relative ratio were determined by X-ray diffractometry. The experimental results indicated that melting temperature and crystallization behavior were influenced by the initial Fe3+/Fe2+ ratio and by the amount of carbon added to the glass batch. For goethite and jarosite glass compositions, decreasing the Fe3+/Fe2+ ratio increased the crystallization rate by favoring magnetite formation. For EAFD glass compositions, the addition of carbon to the batch inhibited chromite–magnetite spinel formation and favored the attainment of an amorphous glassy phase.

Resonant "forbidden" reflections in magnetite.

Abstract: Resonant x-ray scattering was used to investigate electronic fluctuations of the octahedral iron atoms in magnetite. We measured the (002) and (006) ``forbidden'' x-ray diffraction reflections permitted by the anisotropy of the iron anomalous scattering factor. The energy and azimuthal angle dependencies of these reflections, and the polarization analysis, are shown and discussed. The results clearly show $p$ and $d$ iron empty states ordering in magnetite at room temperature. Moreover, the octahedral iron atoms are electronically equivalent in a time scale lower than ${10}^{\ensuremath{-}16}\phantom{\rule{0ex}{0ex}}\mathrm{sec}$. Therefore, magnetite should be considered as an itinerant magnet and not as a fluctuating mixed valence material.

The Lightning Creek Sill Complex, Cloncurry District, Northwest Queensland: A Source of Fluids for Fe Oxide Cu-Au Mineralization and Sodic-Calcic Alteration

Abstract: The Lightning Creek Cu-Au prospect is hosted by a cogenetic suite of plutonic, I-type granitoids. The dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks, including monzogranite and alkali feldspar granite. A series of flat-lying sills are interpreted to be late-stage differentiates, based on their timing, mineralogy, and chemistry. In parts of the prospect there is pervasive sodic-calcic alteration (pyroxene after amphibole, albite after K feldspar and oligoclase) of the plutonic rocks. This alteration predates sill emplacement and is unrelated to veining or fracturing of any kind. The presence of small amounts of carbonate in the altered rocks suggests that the fluids were CO2 bearing. Quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz-monzodiorite, suggesting that the fluids were hot and of magmatic composition. Sodium and Ca were added and K, Fe, Cl, and Cu were stripped during what is interpreted as an autometasomatic event. The sills display considerable textural and mineralogical complexity and evolved from equigranular, quartzofeldspathic rocks (aplites), with magmatic chemistry, to unusual Fe-rich rocks (albite-magnetite-quartz) that exhibit a range of bizarre spherulitic textures. Some of the albite and magnetite in the sills is secondary. Albite forms pseudomorphs after K feldspar (Na-Fe ± Ca alteration) along sill margins and within sills, at the contacts between different textural zones. Halos of disseminated magnetite + clinopyroxene (Fe-Ca ± Na alteration) are developed adjacent to early magnetite veins. Fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500°C and at pressures in excess of 1.5 kbar. The range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase. This magmatic fluid phase was dominated by H2O, CO2, and chlorine and underwent phase separation into a CO2-rich vapor and a hypersaline brine (33–55 wt % NaCl equiv). The hypersaline fluid was enriched in Fe (~10 wt %) and Cu (~1 wt %, PIXE analysis), in addition to Na, K, and Ca. Where this fluid was retained within Fe-rich portions of the sills, it caused Ca-Fe ± Na alteration (pyroxene-albite ± magnetite growth at the expense of quartz). Where the fluid was expelled from the sills, it produced quartz-magnetite ± clinopyroxene ± albite veins (broadly coeval with the early magnetite veins). Although rich in Cu, these granitoid-derived magmatic fluids did not generate significant Cu(-Au) mineralization, perhaps because of the high temperatures involved and/or a lack of reduced sulfur in the fluids or host rock. However, the amount of iron present is estimated (from the aeromagnetic anomaly) to be in excess of 2,000 million tonnes (Mt). A later generation of calcite ± chlorite ± pyrite ± chalcopyrite veins contain traces of Cu-Au mineralization. Fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler (<200°C), more dilute (15–28 wt % NaCl equiv) fluids, perhaps generated by the admixture of a meteoric component. The conclusions reached in this study have implications for understanding the genesis of Fe oxide Cu-Au deposits and related sodic-calcic alteration. The study indicates the potential for CO2-rich granitoid magmas to evolve hypersaline, Fe- and Cu-rich fluids capable of causing intense magnetite veining and Cu(-Au?) mineralization. Autometasomatic sodic-calcic alteration of the granitoids may be an important precursor to mineralization, contributing Fe, K, Cu, and Cl to the magmatic fluids.

Iron oxyhydroxide colloids stabilized with polysaccharides.

Abstract: Neutralization of iron salts in aqueous solutions of κ-carrageenan and cellulose sulfate results in iron oxyhydroxide–polysaccharide hybrid colloids with unusual pH stability up to pH 13 It is shown that both polysaccharides form a tight polymer layer surrounding the inorganic particles, which in the case of κ-carrageenan is cross-linked by helical domains forming a self-assembled nanoreactor The stabilized iron oxyhydroxide particles can undergo further reactions, for example, it is possible by a chemical reaction to produce stabilized magnetite particles Repetition of the loading/neutralization steps in the reaction results in hybrids with iron contents much higher than the stoichiometric balance of iron and functional groups of the polymer (greater than 100% Fe/SO4−) This combination of high iron content with a natural polysaccharide stabilizer makes these colloids interesting for a number of applications, for example, for nutritional purposes or as contrasting agents for tomography

Surface and interface properties of epitaxial iron oxide thin films deposited on MgO(001) studied by XPS and Raman spectroscopy

Abstract: Epitaxial Fe3O4(001) thin films, 5 nm and 25 nm in thickness, have been prepared on MgO(001) substrates by evaporating iron in the presence of oxygen. The XPS and Raman spectroscopy spectra obtained for the as-deposited films confirm the growth of magnetite, as previously observed with reflection high energy electron diffraction, low-energy electron diffraction and AES. The films were annealed in dry air at 400–600 K. For the 25 nm thick film the transformation Fe3O4 → γFe2UBSCO3 is observed at 500 K, in good agreement with the results obtained for bulk magnetite. For the 5 nm thick film annealed at 600 K, Mg/Fe interdiffusion occurs at the interface and a modification in the chemical environment of magnesium is observed on the XPS Mg 2p spectrum. The Raman spectrum of this film is peculiar and cannot be assigned to γFe2O3 or αFe2O3. A new phase, probably the inverse spinel (MgxFe1−x)Fe2O4, is formed. Copyright © 2000 John Wiley & Sons, Ltd.

Mineralogy of the sources for magnetic anomalies on Mars

Abstract: ~~~ Abstract-Recent discovery of intense magnetic anomalies on Mars, which are due to remanent magnetization, requires some explanation for the possible minerals responsible for the anomalous signature. Thermoremanent magnetization (TRM) in single domain (SD) and multidomain (MD) sized magnetite, hematite, and pyrrhotite, all potential minerals, are considered. The intensity of TRM (in 0.05 mT) is in descending order: SD-sized magnetite, SD-sized pyrrhotite, MD-sized hematite, MD-sized pyrrhotite, MD- sized magnetite, SD-sized hematite. The TRM intensity is 50% of the SIRM. Each of these minerals and estimated concentrations of magnetic remanence carriers (assumed to be titanomagnetite) in the Shergotty- Nakhlaxhassigny martian meteorites are used in a thin sheet approximation model to reveal the concentration of each mineral required for the generation of an observed magnetic anomaly (1 500 nT at 100 km altitude) assuming TRM acquisition in a 0.05 mT magnetic field.

Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia

Abstract: — We report in situ measurements of O-isotopic compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized CV chondrites Kaba and Mokoia. In both meteorites, the magnetite and the secondary olivine (fayalite, Fa90–100) have O-isotopic compositions near the terrestrial fractionation (TF) line; the mean Δ17O (= δ17O-0.52 × δ18O) value is about −1%‰. In contrast, the compositions of nearby primary (chondrule), low-FeO olivines (Fa1–2) are well below the TF line; Δ17O values range from −3 to −9%‰. Krot et al. (1998) summarized evidence indicating that the secondary phases in these chondrites formed by aqueous alteration in an asteroidal setting. The compositions of magnetite and fayalite in Kaba and Mokoia imply that the O-isotopic composition of the oxidant was near or somewhat above the TF line. In Mokoia the fayalite and magnetite differ in δ18O by ∼20%‰, whereas these same materials in Kaba have virtually identical compositions. The difference between Mokoia magnetite and fayalite may indicate formation in isotopic equilibrium in a water-rich environment at low temperatures, ∼300 K. In contrast, the similar compositions of these phases in Kaba may indicate formation of the fayalite by replacement of preexisting magnetite in dry environment, with the O coming entirely from the precursor magnetite and silica. The Δ17O of the oxidant incorporated into the CV parent body (as phyllosilicates or H2O) appears to have been much (7–8%‰) lower than that in that incorporated into the LL parent body (Choi et al, 1998), which suggests that the O-isotopic composition of the nebular gas was spatially or temporally variable.

Low‐temperature magnetic properties of magnetite

Abstract: Although several studies have recommended removal of secondary components of magnetic remanence by zero-field cycling from room temperature to a temperature much lower than the low temperature transition for magnetite (about 120 K), the method has not become a standard routine technique. This is partly due to the poor understanding of the behavior of magnetite particles at the low-temperature transition zone. Previous experiments by other researchers have used magnetite powders. In such powders it is always possible to attribute any discrepancy between the results observed and theory to possible existence of magnetostatic interaction effects or existence of elongated particles in samples presumed to contain only equant particles. Such factors need to be eliminated in order to have a better understanding of the low temperature behavior of magnetite particles. Low-temperature magnetic properties of lithographically produced arrays of both interacting and noninteracting cubic magnetite particles as well as those from powder particles have been measured as part of this study. A gradual increase in the amount of saturation isothermal remanent magnetization (SIRM) lost at the Verwey transition Tv with increasing particle size in the pseudo-single-domain size range has been observed. This behavior is consistent with the vortex state domain structure. The grain size dependence of the amount of SIRM lost at Tv is most probably what previous researchers reported as a magnetic memory particle-size-dependent trend. Magnetic memory measured during the cooling and warming process is shown to be a stress-related phenomenon. Such measurements could be useful in assessing the nature of stress in a magnetite sample.